Estimating fugitive dust emission rates using an environmental boundary layer wind … ·...

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Atmospheric Environment 40 (2006) 7668–7685

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Estimating fugitive dust emission rates using an environmentalboundary layer wind tunnel

Jason A. Roneya,�, Bruce R. Whiteb

aDepartment of Mechanical and Aerospace Engineering, University of Colorado,

1420 Austin Bluffs Parkway, Colorado Springs, CO 80918, USAbDepartment of Mechanical and Aeronautical Engineering, One Shields Avenue, University of California, Davis, CA 95616, USA

Received 17 February 2006; received in revised form 15 July 2006; accepted 16 August 2006

Abstract

Emissions from fugitive dust due to erosion of ‘‘natural’’ wind-blown surfaces are an increasingly important part of

PM10 (particulate matter with sizes of 10mm aerodynamic diameter) emission inventories. These inventories are

particularly important to State Implementation Plans (SIP), the plan required for each state to file with the Federal

government indicating how they will comply with the Federal Clean Air Act (FCAA). However, techniques for

determining the fugitive dust contribution to over all PM10 emissions are still in their developmental stages. In the past, the

methods have included field monitoring stations, specialized field studies and field wind-tunnel studies. The measurements

made in this paper allow for systematic determination of PM10 emission rates through the use of an environmental

boundary layer wind tunnel in the laboratory. Near surface steady-state concentration profiles and velocity profiles are

obtained in order to use a control volume approach to estimate emission rates. This methodology is applied to soils

retrieved from the nation’s single largest PM10 source, Owens (dry) Lake in California, to estimate emission rates during

active storm periods. The estimated emission rates are comparable to those obtained from field studies and lend to the

validity of this method for determining fugitive dust emission rates.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Particulate matter; Fugitive dust; Emission rates; Wind tunnel; Aerosols

1. Introduction

1.1. Overview of methodology

The primary goal of the research presented in thispaper is to describe a novel methodology fordetermining fugitive dust emission rates using an

e front matter r 2006 Elsevier Ltd. All rights reserved

mosenv.2006.08.015

ing author. Tel.: +1 719 262 3573;

3042.

ess: [email protected] (J.A. Roney).

environmental boundary layer wind tunnel in thelaboratory. These measurements have not beenpreviously made in the manner described in thispaper and allow for systematic determination ofemission rates that can be used in atmosphericmodels. A case study involving Owens (dry) Lakesoils is used to exhibit the methodology. The effectof surface variability on dust entrainment isimperative in predicting concentration levels in theatmosphere. Thus, this paper may prove beneficialat other sites around the world where wind-blown

.

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ARTICLE IN PRESSJ.A. Roney, B.R. White / Atmospheric Environment 40 (2006) 7668–7685 7669

particulate loadings need to be estimated to addressenvironmental impacts to air quality. The presentwind-tunnel simulation is not meant to be an exactsimulation of all the meteorological conditions atOwens (dry) Lake or any site, but it is meant toisolate the effect of wind-erosion on dust entrain-ment at the surface-air interface. Through winderosion analysis of four soils in a laboratory windtunnel, quantification of the emission rates of dust,specifically, PM10 at Owens (dry) Lake are esti-mated using a novel approach.

A comprehensive discussion of the PM10 problemat Owens (dry) Lake in California and its origin ispresented in our previous paper (Roney and White,2004). Despite recent control strategies, Owens (dry)Lake is still the largest stationary PM10 source in theUSA due to actively blowing soils which producefugitive dust. Due to the need for dust stormmitigation at Owens (dry) Lake, the University ofCalifornia at Davis became active in many of theresearch projects (White and Cho, 1994; Cahill et al.,1996; Kim et al., 2000; White and Roney, 2000; Roneyand White, 2004) aimed at controlling the storms.

1.2. Emission rates and dust suspension

PM10 emission rates at sites with wind erosion arerelated to dust suspension. The emission rate isdefined as the amount of mass suspended per unitarea of surface soil per unit time. In describingsuspended particles, Bagnold (1941) differentiatesbetween ‘‘dust’’ and ‘‘sand’’: ‘‘We can thus definethe lower limit of size of sand grains, withoutreference to their shape or material, as that at whichthe terminal velocity of fall becomes less than theupward eddy current within the average of thesurface wind’’. Typically, the aerodynamic diameterat this limit is estimated at 50 mm; dusto50 mm, andsand 450 mm (Lancaster and Nickling, 1994). Inaddition, long-term suspension of dust is estimatedfor diameters o20 mm (Pye, 1987); thus, PM10 hasthe potential for long-term suspension.

Suspension and production of dust occursthrough two mechanisms; abrasion and throughthe direct action of the wind on the surface.Abrasion can occur through saltation (impact bysand-sized particles hopping along the surface)while wind forces cause both wind shear andturbulent shear (Braaten et al., 1993) that maysuspend particles. Once in suspension, dust istransported by two means; horizontal advection,and turbulent diffusion. Both suspension and

transport properties are important when estimatingthe emission rate of a given surface. Dust mayoriginate from an upwind source not related to thesurface (transport) or be entrained or transportedlocally from the surface (turbulent diffusion andadvection). A true estimation of the emission rate ofa surface must involve a subtraction of the non-related advection component. The upwind advec-tion component is known for the wind-tunnel studypresented in this paper, thus, a better approxima-tion for the emission rate is made using the controlvolume approach for a specific soil.

To exemplify this last point, consider a typicalvertical dust flux calculated with a field tower aspresented in numerous studies (Gillette et al., 1972;Gillette and Goodwin, 1974; Gillette and Walker,1977; Cahill et al., 1996; Gillette et al., 1997a;Niemeyer et al., 1999). The vertical flux formulationresults from letting the sedimentation and theadvection term go to zero in the governing transportequation and assuming the remaining mechanism isvertical diffusion or flux of dust. The vertical flux ofdust Fa is then,

Fa ¼ �ku�zqc

qz, (1)

where c is the concentration of dust, k is the vonKarman constant, u* is the friction velocity, and z isthe height from the surface. A negative concentra-tion gradient results in a flux of dust away from thesurface. This simplification for the flux is only validover idealized flat terrain where the turbulence inthe boundary layer is adequately described by u*.The vertical flux equation can be written discretelyto evaluate individual measurement from a meteor-ological tower as follows:

Fa ¼ �ku�z2Caðz2 þ Dz; xÞ � Caðz2 � Dz; xÞ

2Dz, (2)

where z2 represents a vertical reference point fromwhich concentrations Ca are measured at equallyspaced vertical distances Dz apart. With this methodonly two vertical measurement locations are neededin the field to make an estimation of the verticalflux. This formulation, however, is highly dependenton vertical gradients of concentration. Large emis-sions local to the monitoring tower suggest a largesource of potential emissions; however, emissionsupwind of these measurements could also influencethe result. Gillette and Goodwin (1974) note a casewhere a negative aerosol flux was observed usingthis methodology, possibly, due to a ‘‘higher rate of

ARTICLE IN PRESSJ.A. Roney, B.R. White / Atmospheric Environment 40 (2006) 7668–76857670

erosion upwind of the sampling tower than near thetower’’. Thus, in this case, the ineffectiveness of theformulation results from a failure of the experi-ments to satisfy the assumption that advection isnegligible.

The vertical flux approach to emissions isubiquitous in the current literature, both in field,modeling, and wind-tunnel studies (Chepil, 1945;Cowherd and Ono, 1990; Stetler and Saxton, 1996;Borrmann and Jaenicke, 1987; Fairchild andTillery, 1982). In several of these studies, thehorizontal sand saltation flux has been measuredas well as a means to quantify the effect of saltationon the vertical flux of dust. In this paper we will usethe vertical flux of dust to provide a comparisonbetween our data and existing field data.

2. Experimental methods

2.1. Soil collection rationale and preparation

The details of soil collection rationale at Owens(dry) Lake and soil preparation are given in Roneyand White (2004). In brief, Owens (dry) Lake areaspreviously shown to be sources of high emissionwere chosen for wind-tunnel soils testing. An effortto collect loose elastic soils of the same soil textureand similar size distributions at each site was madeby collecting only those soils that were loose, sandsor those under the thin crusts on the playa. TheGPS coordinates, map locations, and descriptionsof the soils are given in Roney and White (2004).For identification in the laboratory, the four soilswere given names related to the location ofcollection as shown in Table 1. Nearly 5 t of soilfrom these locations was transported back to theUniversity of California at Davis for the laboratorywind tunnel testing. Once at the University ofCalifornia at Davis, they were prepared in a mannerto represent the most emissive conditions; they wereair dried, and if present, major aggregate clumpingwas eliminated with a coarse sieve. Random

Table 1

Global positioning system (GPS) locations for the four soils collected

Desig. Description Suspe

Soil #1 Old Pipe Line, Loamy Sand Emis

Soil #2 North Sand, Sand ‘‘San

Soil #3 Dirty Socks Dune, Sand ‘‘San

Soil #4 UCD Fence, Sandy Loam Emis

samples from each of the soil types were tested forchemical nature and texture as well as size distribu-tions. The size distributions were consistent for eachsoil type and an average size distribution for eachsoil is shown in Roney and White (2004).

2.2. Wind tunnel

All measurements were made in the saltationwind tunnel (SWT), an environmental boundarylayer wind tunnel at the University of California atDavis (Kim et al., 2000). This open-circuit windtunnel is designed to simulate particle flows orsaltation movement, and thus, is ideal for simulat-ing the emission of dust from crustal surfaces suchas Owens (dry) lake. The inlet of the wind tunnelhas an array of flow straightening tubes to eliminateany large-scale turbulence resulting from objects inthe surrounding room allowing the flow to developnaturally in the wind tunnel. Following the inlet, thetunnel has a 5m section to develop a turbulentboundary layers characteristic of the flow near thesurfaces of desert playas. In this section, pebblesattached to the bottom surface were evenly spacedbut randomly oriented such that a well-developedtwo-dimensional turbulent boundary layer formedprior to impinging on the 5m long soil bed. Withinthe sections containing the soil of interest, theboundary layers were closely matched due to similarroughness characteristics. To maintain an evendepth of soil of approximately 30mm, a troughrunning down the centerline of the wind-tunnel withdimensions 0.025m� 0.30m was used. The sidesurfaces were covered with sand paper to match theroughness of the soils (Fig. 1). This depth of soilallowed for about 5 to 10 min of testing at lowerspeeds before appreciable amounts of soils werelost. Because of the soil loss, the maximum velocitytested in the wind tunnel could not exceed values of14.0m s�1. Lastly, the diffuser section opened to theoutside atmosphere expelling any suspended dust orsand.

at Owens (dry) Lake

cted type GPS lat. GPS long.

sive soil 36128.808N 117154.649W

d’’ 36129.194N 117154.655W

d’’ 36120.391N 117157.681W

sive soil 36121.411N 117157.467W

ARTICLE IN PRESS

Fig. 1. Measurement set-up for testing emissions of loose soils: (a) a schematic; and (b) a photograph showing the soil bed which the

emissions were measured over.

J.A. Roney, B.R. White / Atmospheric Environment 40 (2006) 7668–7685 7671

The wind-tunnel measurement instrumentationconsisted of two TSI DustTrakss to measure PM10

aerosol concentrations (‘‘fugitive dust’’), a traversingtotal pressure probe to measure the vertical velocity,a Pitot-static probe to measure the mean free-streamvelocity Uref (located at 0.22m above the surface),and stackable isokinetic sand traps (White, 1982) tomeasure the horizontal sand saltation flux. At thispoint it is important to state the TSI DustTraks isnot a federal reference method (FRM) or equivalentmethod for PM10; however, for this study, the GreatBasin Unified Air Pollution Control District

(GBUAPCD) compared their DustTraks to thetapered element oscillating microbalance (TEOM)samplers (an equivalent method) at three sites atOwens Lake, and consistently, the DustTraks readapproximately 50% of the TEOM value. In Ono etal. (2000), a comparison was performed between theTEOM and FRM samplers at Owens (dry) Lake.The FRM sampler values ranged from 60% to 106%of the TEOM values, so the factory calibration of theDustTraks with Arizona Test Dust seems to be areasonable approximation for dust-types at Owens(dry) Lake.


2.3. Loose soil emission measurements

Once the boundary layer flows were sufficientlyestablished in the wind tunnel, the loose soilemission rates were measured for values above thethreshold friction velocity u*t. With the soil in place,emission rates were obtained experimentally byinstantaneously measuring velocity and concentra-tions by simultaneously vertically traversing twoDustTrakss and a total pressure probe at 2.65 and4.38m from the leading edge of the soil bed for a setof the same 10 heights (Fig. 1). The DustTrakss

recorded concentration at each location in mgm�3

with the data acquired by an A/D board attached toa PC at a sampling rate of 1Hz. At each traversingheight measurements were taken for 10 s; thesesampling times were used to give a sufficient numberof measurements while allowing the test to becompleted in less than five minutes maintaining a‘‘steady’’ process. To obtain an emission rate, acontrol volume analysis was used between thevarious inlet and outlet measurements (Fig. 2).The emission rate was defined as the mass emitted ina unit area per unit time, [ML�2 T�1] or[mgm�2 s�1]. The control volume was defined asWb�Lb�Ht where Wb is the width of the soil bed,Lb is the length of the soil bed or length betweenprobes, and Ht is the height of the tunnel. The massflux out of the control volume was defined as _mout

and the mass flux in as _min, where ‘‘in’’ and ‘‘out’’are relative to the control volume. The emission rate_E for the control area, Ab ¼Wb�Lb, was found byapplying a mass balance on the control volume:

_E ¼1

Abð _mout � _minÞ. (3)

Since the aerosol sampler measures concentra-tions, the mass flux rates were described as a

Fig. 2. Schematic representing the method of determinin

function of concentration:

_mout ¼

Z Ht

0

coutuoutWb dz, (4)

_min ¼

Z Ht

0

cinuinWb dz, (5)

where u and c represent the velocity and concentra-tion 10-s averages. The c-subscripts represententrance (‘‘in’’) and exit (‘‘out’’) locations. z is thedirection away from the surface. The revisedemission rate equation is

_E ¼1

Lb

Z Ht

0

ðcoutuout � cinuinÞdz. (6)

For a simplified analysis, emissions upwind of thesoil bed were assumed to be zero. This assumption isbased on measurements of dust concentrations inthe inlet of the wind tunnel that showed that theincoming air had negligible amounts of dust relativeto the measurements above the soil bed. Likewise,the velocity profiles are assumed equivalent at theentrance and exit of the control volume. Testingshowed that velocity profiles measured simulta-neously at 2.65 and 4.38m were similar. Once thevelocity and concentration profiles were obtained,emission rates were calculated from Eq. (6).Regression curves were fit to each PM10 concentra-tion profile and velocity curve. The product of thetwo was integrated numerically to obtain anemission rate per unit area _E. This procedure wasrepeated for numerous free-stream wind-tunnelvelocities ranging from 8.5 to 14.0m s�1 andcorresponding to different u* friction velocity valuesranging from 0.35 to 1.1m s�1 for each soil. Typicalturbulent boundary layer heights d at the measure-ment locations ranged from 22 to 25 cm. Theemission rate measurements were replicated for

g loose soil emission rates with a control volume.


ranges of u* for each soil; the number of replicationsvaried from one to four. However, data-acquiredmeasurements were made during each test givingmultiple measurements of individual quantitiesduring the test as well. Determination of theroughness height z0, the friction velocity u*, andthe Coefficient of Drag Cd of each surface isdiscussed in Roney and White (2004) in detail.Both z0 and u* are determined by using mixinglength theory for turbulent boundary layers thathave roughness in combination with experimentalresults; z0 is determined from velocity profilesprior to threshold, and u* is related to the slope ofthe profiles. The Cd of the surface is calculatedfrom the ‘‘prethreshold’’ linear relationship of u*versus the reference free-stream velocity Uref. Theconcentration profile results also contain informa-tion on the vertical flux of dust Fa which wasevaluated as previously shown in Gillette andWalker (1977), and Borrmann and Jaenicke(1987). Fa is compared to the emissions data inother published studies.

The sand flux was obtained simultaneously forseveral of the experiments by using 15–21 sand trapsstacked on top of each other. Each of these traps was2.0 cm high by 1.0 cm wide and had a frontalsampling area of 0.0003m2. The traps were placedat the exit of the test section near the diffuser with thefrontal area perpendicular to the wind velocity. Thehorizontally moving mass greater than 40mm aero-dynamic diameter was collected in the traps. Thetraps are approximately isokinetic samplers as the airmoves freely through the sampler at approximatelythe same rate as the wind velocity (White, 1982). Thetime ts of collection was recorded, and the mass ineach trap was weighed. A sand flux was obtained inthe following way at each height:

qi ¼mi

tsAi

, (7)

where mi is the mass collected in each sand trap ateach location, Ai is the frontal area of the trap, and tsis the time of collection. Once the sand flux had beenobtained, a total flux was obtained with the followingequation:

Q ¼X21i¼1

qihi, (8)

where hi is the height of the trap. This sand flux wasan indication of the saltation rate for various windvelocities for each soil.

In addition, the ratio of horizontal PM10 flux tototal horizontal mass flux was obtained from theemission rates and sand flux rates. Gillette et al.(1997a) use the ratio of the vertical flux of dust Fa tothe total horizontal mass flux to describe fieldmeasured emissions at Owens (dry) Lake. Acomparison between the wind-tunnel measuredvertical flux of dust Fa to the total horizontal massflux ratio was also compared with this field study.

3. Results

3.1. PM10 loose soil emission rates

Velocity profiles were obtained for each saltationcase during emissions testing. Above thresholdconditions, there is no longer a focus of profiles toz0, since movement of the soil causes an increasedeffective roughness due to saltating particles andripples. For each increase in free-stream velocity,there is a different z-intercept which is denoted z0

0.However, there is a new focus of the velocity profilesdenoted as z0 with a corresponding off-set velocityUz0. Bagnold (1941) found a similar result for his

velocity profiles in his experiments on sand move-ment. A sample velocity profile plot for one of thefour soils is shown in Fig. 3.

Simultaneously, PM10 concentration profiles wereobtained for several cases at 2.65 and 4.38m fromthe beginning of the soil bed. Typical data for oneexperimental test are shown in Fig. 4. The data werecurve-fit to expedite the integration, with the typicalcurve-fit for the concentration profiles having apower-law form C ¼ az�b where C is the concentra-tion and ‘‘a’’ and ‘‘b’’ are fitting coefficients (anoccasional fit was of the form C ¼ d0e

�d1z where d0and d1 are fitting coefficients). Likewise, the velocityprofiles were fit with ‘‘law-of-wall’’ type fits typicalof turbulent boundary layer profiles. These fits areshown in Fig. 5. The integration as specified inEq. (6) was done with a discrete numericaltechnique (trapezoidal rule) and the limits ofintegration taken between 0.01 and 0.5m withDz ¼ 0.01m. For the concentration profiles, extend-ing the curve-fit beyond the region of the data nearthe surface posed the possibility of introducingerroneous mass, since the true tendency very nearthe surface (less than 0.01m) is typically for theprofile to saturate and not follow the power-law fit.Using these results and Eqs. (3)–(6) emissions ratesand horizontal PM10 fluxes were calculated.

ARTICLE IN PRESS

Pipe Line Soil (Soil #1)Saltation Velocity Profiles

Wind Speed (m/s)

0 5 10 15

z (m

)

10-5

10-4

10-3

10-2

10-1

100

Uz'

z'

z'0 varies for each profile

Mass Loading Effect

Fig. 3. Sample velocity profiles for the cases when the soil is

being blown and transported. For each increase in free-stream

velocity, there is a different z-intercept which is denoted z00, and

there is a new focus of the velocity profiles denoted as z0 with a

corresponding off-set velocity Uz0.

Old Pipe Line (Soil #1), Uref = 9.8 m/s

PM10 (mg/m3)

10-1 100 101 102

z (m

)

0.01

0.10

1.00

PM10 In (2.65 m)

PM10 Out (4.38 m)

C2.65=0.0767z-1.0576

C4.38=0.0959z-1.4795

Upper Integration Limit

Lower Integration Limit

Emissions

Fig. 4. The simultaneous PM10 concentration profiles for Soil #1

at Uref ¼ 9.8m s�1 for both the 2.65m fetch and the 4.38m fetch

distance. There is a significant gain in the concentration levels

between the two probes.

Linear Plots of Integrated ComponentsUref = 9.8 m/s, x = 4.38 m

Velocity (m/s)

5 8 9 10 11

z (m

)

0.10

0.20

0.30

0.40

0.50

PM10 (mg/m3)

0 20 40 60 80 100

Concentration, ciVelocity, ui

C4.38=0.0959z-1.4795

u=1.20∗ln(z/0.0001)

6 7

Fig. 5. Simultaneous velocity and concentration profiles ob-

tained at x ¼ 4.38m in the wind tunnel. These profiles are then

used to obtain an emission rate for the wind-tunnel measure-

ments.

J.A. Roney, B.R. White / Atmospheric Environment 40 (2006) 7668–76857674

For these cases, PM10 horizontal flux values at x ¼

2:65m could be compared to values at x ¼ 4.38m ineffect looking at the fetch effect on emissions. Two

different types of results occur in this comparison;first, those where there is distinctly increasing amountsof PM10 along the length of the soil test bed between2.65 and 4.38m (Fig. 4), and second, those where thePM10 levels remain about the same or even decreaseslightly. For the majority of the test cases at thehighest velocities, the difference in horizontal massbetween 2.65 and 4.38m locations is small (Fig. 6).Thus, if we take the control volume between 2.65 and4.38m there is no emission rate or a negative emissionrate. At first, this posed a perplexing question asprevious studies (Gillette et al., 1996) suggests that dueto the Owen effect the mass should increase substan-tially along the fetch.

An initial conceptual model based on these observa-tions was developed indicating how these situationsmay arise (Fig. 7): at lower velocities the horizontalmass gain is approximately linear with increasing fetch;however, at higher velocities the wind tunnel height andnear-surface mass saturation becomes a limitation onhow much additional mass can become entrained. Thesaturation effect is a result of high mass concentrationsof fine particles in the wind-tunnel air at themeasurement location both at the surface and in the

ARTICLE IN PRESS

UCD Fence Soil (Soil #4), Uref = 12.7 m/s

PM10 (mg/m3)

10-1 100 101 102

z (m

)

0.01

0.10

1.00

PM10 In (2.65 m)

PM10 Out (4.38 m)

C2.65=0.2330z-1.1910

C4.38=0.1600z-1.1950



Fig. 6. The simultaneous PM10 concentration profiles for Soil #4

at Uref ¼ 12.7m s�1 for both the 2.65m fetch and the 4.38m fetch

distance. In this case, the concentration levels between the two

probes remain about the same. The lines are the curve fits to the

concentration profiles.

Fig. 7. A conceptual model for determining the emission rates in

the wind tunnel is presented. The wind-tunnel size (the height) is

considered a limitation on the naturally evolving emission for the

highest speeds. The emissions rates should thus if possible be

calculated in regions were there is nearly linear gain in emissions.

Pipe Line Soil (Soil #1)Horizontal PM10 Fluxes

x, Soil Fetch Distance (m)

0 2 4

FH, H

oriz

onta

l PM

10 F

luxe

s (m

g/m

s)

0

10

20

30

40

50

60

70u∗ = 0.40 to 0.49u∗ = 0.50 to 0.59u∗= 0.60 to 0.69

u∗ = 0.70 to 0.79

u∗ = 0.80 to 0.89

1 3 5

Fig. 8. A sample experimental study of the horizontal flux shows

that the emissions follow very closely to the conceptual model.

Plots of the horizontal flux for the other three soils show a

comparable result.


free-stream air. The air may be saturated in the wind-tunnel compared to natural conditions for a particularlocation, because we are confining the emissionmechanism between the wind-tunnel walls duringstrong emissive conditions. In ‘‘nature’’ we would seecontinued upward diffusion. With near-surface satura-

tion, the turbulent diffusion mechanism near thesurface decreases due to increase mass loading nearthe surface; this may occur in ‘‘nature’’ as well. Also,with deposition the concentrations may drop. Nosignificant deposition was observed on the wind-tunnelwalls as strong advection transported the particlesdownstream; however, decreases in the near surfaceentrainment mechanism may have allowed depositionon the surface. So, saturation may occur for tworeasons: (1) significant mass at the surface affecting thediffusion mechanism and/or (2) the wind-tunnel heightlimits the amount of upward mixing during strongturbulent diffusion. Experimental results verifying thisconceptual model are shown in Fig. 8. Based on thisconceptual model, the emission rates were calculated inthe linear region or between the beginning of soil bedand the position of the last profile.

For all the loose soil emission measurements, thecontrol volume was taken as the beginning of thebed x ¼ 0–4.38m; Lb ¼ 4.38m in Eq. (6) where _mout

can be calculated and _min ¼ 0 at the beginning ofthe bed (x ¼ 0). This method of calculation providesan average emission rate over the entire soil bed asgiven in Tables 2–5. Likewise, a similar calculationwas performed for cases where concentrationprofiles were measured at x ¼ 2:65m such that the


exit mass is calculated at x ¼ 2.65m and _min ¼ 0.The results for these cases are given in Tables 6–9.

The values shown in the tables are averageestimates for all the studies conducted with a loose‘‘dry’’ soil. The highest emission rates measuredwere 20 000–25 000 mgm�2 s�1 for the ‘‘emissivesoils’’, Soil#1 and Soil#4. However, for the UCDFence Soil (Soil #4) the 25 000 mgm�2 s�1 seems tolie outside the realm of the rest of the data for thatsoil-type. However, homogeneity of soils withinsample locations is assumed, and it possible that thesoil used in this test differed slightly. Over all, thePipe Line Soil (Soil #1) from the North appears tobe the most ‘‘emissive’’ of the four soil types. Asample plot showing the estimated emission ratesand trends for the North Soils is shown in Fig. 9.

Table 2

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the Pi

measurements

Desig. u* (m s�1) Uref (m s�1) u(10m) (m s�1

Soil #1 0.47 9.0 12.7

Soil #1 0.48 9.6 13.4

Soil #1 0.49 10.4 14.0

Soil #1 0.50 9.8 13.8

Soil #1 0.57 10.3 15.1

Soil #1 0.61 10.6 15.4

Soil #1 0.71 11.3 17.2

Soil #1 0.76 11.2 18.1

Soil #1 0.78 11.3 18.1

Soil #1 0.80 11.32 18.5

Soil #1 1.12 14.2 24.3

aData points marked are unusually high or low, and thus, were not

Table 3

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the N

measurements

Desig. u* (m s�1) Uref (m s�1) u(10m) (m s�1)

Soil #2 0.47 9.1 13.2

Soil #2 0.48 8.9 13.0

Soil #2 0.53 10.3 14.4

Soil #2 0.53 10.1 14.4

Soil #2 0.59 10.6 15.7

Soil #2 0.59 10.1 14.6

Soil #2 0.65 11.3 16.7

Soil #2 0.76 11.6 17.9

Soil #2 0.85 12.9 19.9

Soil #2 0.98 13.0 21.5

Soil #2 1.00 13.0 21.6

aData points marked are unusually high or low, and thus, were not

Calculations likes those presented in this plot areshown for the other soils in the tables.

The most surprising result is that Dirty SocksDune Sand (Soil #3) contains a high amount of PM10

and is nearly as emissive as the ‘‘emissive’’ soils at thelower u* values. At Owens (dry) Lake, this may resultfrom the deposition of PM10 in the sand due tonortherly wind storm events in which large amountsof PM10 from the UCD Fence Soil (Soil #4) fall-outover the sand and become integrated among thegrains of sand. This result indicates that Soil #3could potentially be a large source of emissionsduring storms on Owens (dry) Lake. At the higher u*values the Dirty Socks Sand emissions levels off,possibly, due to sand ripples offering protection thatlimits increases in the dust emissions.

pe Line Soil (North Soil) at x ¼ 4.38m from the wind-tunnel

) _E (PM10) (mgm�2 s�1) Fa (PM10) (mgm

�2 s�1)

81.5 74.8

33.0 20.0

103.0 109.8

2600 5280

176.0a 281a

5920 9290

4580 8980

5040 10 130

15 200a 27 340a

7560 14 050

19 420 41 170

used in the plots.

orth Sand (North Soil) at x ¼ 4.38m from the wind-tunnel

_E (PM10) (mgm�2 s�1) Fa (PM10) (mgm

�2 s�1)

52.0 86.6

24.9 23.7

137 200

103 246

393 493

384 1090

449 514a

1200 3330

1180 3020

1280a 3810

3388 7020

used in the plots.

ARTICLE IN PRESS

Table 4

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the Dirty Socks Sand (South Soil) x ¼ 4.38m from the wind-tunnel

measurements

Desig. u* (m s�1) Uref (m s�1) u(10m) (m s�1) _E (PM10) (mgm�2 s�1) Fa (PM10) (mgm

�2 s�1)

Soil #3 0.56 9.9 14.3 25.2 34.6

Soil #3 0.58 9.0 14.3 48.5 8.9

Soil #3 0.61 9.9 15.2 1120 2370

Soil #3 0.67 11.2 16.7 1620 2010

Soil #3 0.68 11.5 17.5 1920 2890

Soil #3 0.75 11.4 18.1 783a 862a

Soil #3 0.79 12.9 19.4 2660 3190

Soil #3 0.84 12.6 19.1 2580 3890

Soil #3 1.01 12.5 21.6 3640 7090

Soil #3 1.02 13.0 22.1 3015 5290

Soil #3 1.09 13.6 23.4 3731 6920

aData points marked are unusually high or low, and thus, were not used in the plots.

Table 5

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the UCD Fence Soil (South Soil) at x ¼ 4.38m from the wind-tunnel

measurements


�2 s-1)

Soil #4 0.35 8.5 11.1 35.4 5.5

Soil #4 0.42 9.9 13.2 223 138

Soil #4 0.49 10.5 14.3 2230 1440

Soil #4 0.51 8.9 13.2 20.0a 22.3a

Soil #4 0.56 11.6 16.3 1871 2400

Soil #4 0.63 13.1 18.7 1342 1890

Soil #4 0.67 11.7 17.0 497a 1,280

Soil #4 0.69 11.6 17.6 3637 8490

Soil #4 0.69 13.0 19.0 2434 3770

Soil #4 0.70 13.7 20.0 3055 2270

Soil #4 0.71 12.7 19.0 2550 4140

Soil #4 0.75 12.7 19.1 9480a 21 460a

Soil #4 0.86 13.9 21.7 13 740a 21 990a

Soil #4 0.90 13.8 22.0 9850 21 950a


Table 6

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the Pipe Line Soil (North Soil) at x ¼ 2.65m from the wind-tunnel

measurements


�2 s�1)

Soil #1 0.47 9.0 12.7 22.3 —

Soil #1 0.50 9.8 13.8 1030 498

Soil #1 0.61 10.6 15.4 3730 2890

Soil #1 0.78 11.3 18.1 24 800a 35 670a

Soil #1 0.80 11.32 18.5 14 200 16 600



Unusually low or high points were systematicallyeliminated to form clearer plots and are marked byan asterisk in Tables 2–9. These data points are left

in the tables for completeness as they may besuggestive of the variability within some of the soiltypes. In addition, all the Tables include an

ARTICLE IN PRESS

Table 7

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the North Sand (North Soil) at x ¼ 2.65m from the wind-tunnel

measurements


�2 s�1)

Soil #2 0.44 9.2 12.2 95.6 34.0

Soil #2 0.48 8.9 13.0 38.3 11.0

Soil #2 0.53 10.0 14.2 370 273

Soil #2 0.59 10.1 14.6 400 732

Soil #2 0.69 11.8 17.2 960 837

Soil #2 0.76 11.6 17.9 1680 3780a

Soil #2 0.85 12.9 19.9 1230 1690

Soil #2 0.90 12.8 20.6 1250 1740

Soil #2 0.98 13.0 21.5 1660 3360


Table 8

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the Dirty Socks Sand (South Soil) at x ¼ 2.65m from the wind-tunnel

measurements


2 s�1)

Soil #3 0.32 8.4 10.4 9.2 0.5

Soil #3 0.40 9.3 11.9 22.7 28.3

Soil #3 0.58 9.0 14.3 89.2 19.1

Soil #3 0.61 9.9 15.2 670 975

Soil #3 0.61 11.1 16.3 790 603

Soil #3 0.68 11.5 17.5 3200 4390

Soil #3 0.71 12.8 18.9 3770 5000

Soil #3 0.75 12.9 19.4 4330 6310

Soil #3 0.79 12.9 19.4 5190 6260

Soil #3 0.84 12.6 19.1 4460 6160

Table 9

Calculated ‘‘loose’’ soil emission rates and vertical fluxes for the UCD Fence Soil (South Soil) at x ¼ 2.65m from the wind-tunnel

measurements


�2 s�1)

Soil #4 0.35 8.5 11.1 14.7 3.0

Soil #4 0.39 9.4 11.9 174 6.2

Soil #4 0.42 9.9 13.2 240 8.7

Soil #4 0.48 9.9 13.6 379 216

Soil #4 0.49 10.5 14.3 1720 384

Soil #4 0.52 11.0 14.7 1120 440

Soil #4 0.56 11.6 16.3 2980 2926

Soil #4 0.63 13.1 18.7 2060 1860

Soil #4 0.66 12.2 17.0 2350 3190

Soil #4 0.69 13.0 19.0 2660 2500

Soil #4 0.70 13.7 20.0 3670 3340

Soil #4 0.71 12.7 19.0 6070 5930

Soil #4 0.76 12.9 19.2 4240 7110

Soil #4 0.78 13.1 19.5 4140 5630


equivalent wind value for 10 m heights. Thisvelocity is calculated by an extension of the law-of-the-wall boundary layer profiles.

All of the soils at the highest wind-tunnel speedsrapidly formed ripple beds. The wind forms ridgesof sorted particles that are well established very


quickly in the tunnel. Since these beds are formed sorapidly, there is no need to consider their timedependence and their effect on the flow after theinitial sorting. The time constant for the initialsorting was estimated at 100 s from start-up of thewind tunnel to formation of ‘‘steady’’ ripples.Analysis shows that some of the initial points inthe profiles were taken before this time constant wasexceeded. However, these points usually appearederroneous or saturated in the profiles and the fits didnot include these data points; therefore, the profilesseem to be an accurate representation of the

Owens Lake Soil Emission RatesNorth Soils (x = 4.38 m)

(u∗∗ - u∗t)2 (m2/s2)

0.0 0.2 0.4 0.6 0.8 1.0

PM

10 E

mis

sio

n R

ate

(μg

/m2 s

)

0

5000

10000

15000

20000

25000

North Sand (Soil #2)

Pipe Line Soil (Soil #1)

R2 = 0.94

R2 =0.95

E = 28 800(u∗-u∗t)2

E = 6500(u∗-u∗t)2

Fig. 9. A plot for the north soil emission rates showing the rates

are related to the friction velocity squared (or the shear velocity).

Comparable plots were made for the other soil cases as well. The

error bars represent the uncertainty associated with the measure-

ment and the calculation.

Table 10

Calculated emission rates and vertical fluxes for enhanced saltation fo

measurements

Desig. Descrip. u* (m s�1) Uref (m s�1) u(

Soil #2 over Soil #1 North Sheet 0.44 9.2 12




Soil #3 over Soil # 4 South Sheet 0.32 8.4 10





‘‘steady-state’’ emission process with well-formedripples. A caution then is that the profiles presentedin this paper may not capture an initial ‘‘blow-off’’of dust which is highly time dependent, and couldpossibly be another source of high, however, non-sustained emissions.

3.2. PM10 emissions with upwind saltation

The analysis of the last section was reproducedwith Eqs. (3)–(6) for cases with ‘‘naturally’’ occur-ring upwind saltation: the first bed of ‘‘emissivesoil’’ in the wind tunnel was replaced with a bed ofnatural sand particles from Owens (dry) Lake,either with the north sands or south sands closestto the emissive soil location. For the south sandblowing over the ‘‘UCD Fence Soil’’, the simulationwas termed ‘‘South Sheet’’, and for the north sandblowing over the ‘‘Pipeline Soil’’, the simulation wastermed ‘‘North Sheet’’. For these cases, the controlvolume was taken as the volume between the 2.65mprobe and the 4.38m probe. The integrated profilesat each location were subtracted from each otherand divided by Lb ¼ 1.73m, in effect capturingthose emissions that were due to the soil bombard-ment by sand particles and not those pertaining tothe sand movement before the emissive bed. In thisway, the advected upwind dust concentrations fromthe sand were eliminated in the calculation. Theresults are shown in Tables 10 and 11. Table 11shows the emissions at x ¼ 2.65m from the upwindsands.

Saltation has marked effect on the emission rateas exemplified in the North Sheet simulation shownin Figs. 10 and 11 where there is an abrupttransition from sand emissions to soil emissions.This enhancement is likely due to sand particles

r the north and south soils at x ¼ 4.38m from the wind-tunnel

10m) (m s�1) _E (PM10) (mgm�2 s�1) Fa (PM10) (mgm

�2 s�1)

.2 239 231

.2 1975 2654

.2 11 600 11 900

.6 27 600 30 520

.4 3.58 1.5

.9 23.2 126

.3 2920 2286

.9 5640 7297

.4 4620 8313

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Table 11

Calculated emission rates for enhanced saltation for the north and south soils at x ¼ 2.65m from the wind-tunnel measurements

Desig. Descrip. u�ðms�1Þ U ref ðms�1Þ uð10mÞ ðms�1Þ _EðPM10Þðmgm�2s�1Þ

Soil #2 over Soil #1 North Sand 0.44 9.2 12.2 95.6

Soil #2 over Soil #1 North Sand 0.53 10.0 14.2 370



Soil #3 over Soil # 4 Dirty Socks Sand 0.32 8.4 10.4 9.2

Soil #3 over Soil # 4 Dirty Socks Sand 0.40 9.3 11.9 22.7

Soil #3 over Soil # 4 Dirty Socks Sand 0.61 11.1 16.3 790



North Sheet SimulationHorizontal PM10 Fluxes

x, Soil Fetch Distance (m)

0

FH, H

oriz

onta

l PM

10 F

luxe

s (m

g/m

s)

0

10

20

30

40

50

60

70u∗ = 0.44u∗ = 0.53u∗ = 0.69u∗ = 0.90

1 2 3 4 5

Fig. 10. A sample experimental study of the horizontal flux for

the enhanced saltation studies of the north soils. A sharp

delineation between the upstream sand and the pipe line soil

emissions can be seen in this plot. In addition, the emissions

become enhanced due to sand particles impacting the pipe line

soil.

North Sheet Simulation,Uref = 12.8 m/s

PM10 (mg/m3)

10-1 100 101 102

z (m

)

0.01

0.10

1.00

PM10 In (2.65 m)

PM10 Out (4.38 m)



C2.65=0.0534z-1.1797

C4.38=0.2927z-1.5399

Emissions

Fig. 11. The simultaneous PM10 concentration profiles for north

sand saltating over pipe line soil at Uref ¼ 9.8m s�1 for both the

2.65m fetch and the 4.38m fetch distance. There is a significant

gain in the concentration levels between the two probes as shown

corresponding to significant emissions.


impacting the soil with ballistic trajectories. Theseimpacts eject PM10 into the air allowing it to moreeasily be entrained into the turbulent flow and betransported upward. In addition, agglomerations inthe soil are likely abraded into smaller moreemissive particles by the coarser saltating sand. InFig. 12, the emission rates are plotted as before. Inthe plot, the sustained dust threshold frictionvelocity u*t as calculated and presented in Roneyand White (2004) are subtracted from the frictionvelocity u*. The combined term is then squared such

that the emissions begin approximately at zero. Theenhancement in emission rates by introducingsaltating sand upstream of the emissive soils isindicated by the trend lines in Fig. 12. The two setsof soils are presented with regards to their soiltexture as well, and the soil texture appears to be astrong indicator of emissions as well.

3.3. Vertical dust fluxes

Eq. (2) was used to estimate a vertical flux of dustFa from two points in each profile at both thex ¼ 2.65 and 4.38m fetch locations. A summary of

ARTICLE IN PRESS

Owens Lake Soil Emission Rates

(u∗ - u∗t)2 (m2/s2)

0.0 0.2 0.4 0.6 0.8 1.0

PM

10 E

mis

sion

Rat

e (μ

g/m

2 s)

0

5000

10000

15000

20000

25000

30000

35000

Loamy Sandand

Sandy Loams

Sands

SimulationSheet

Dirty Socks Sand (Soil #3)UCD Fence Soil (Soil #4)

North Sand (Soil #2)

Pipe Line Soil (Soil #1)

North Sheet (#2 over #1)

South Sheet (#3 over #4)

E = 75 000(u∗-u∗t)2

E = 7900∗(u∗-u∗t)2

E = 27200∗(u∗-u∗t)2

Fig. 12. The PM10 emission rates as a function of effective wind shear velocity squared for all the soils is shown above. The data is sorted

by soil-type showing differences between the sands and ‘‘loamy’’ soils. Furthermore, the enhanced saltation (the sheet simulations)

produces even more emissions. The error bars represent the uncertainty associated with the measurement and the calculation.


the values is given in Tables 2–9. Trends for thevertical fluxes are similar to those shown in Fig. 9for the emission rate estimates. The similarity intrends indicates the vertical flux of dust is directlyrelated to the horizontal flux of dust measured inthe wind tunnel.

3.4. Sand fluxes

Total sand fluxes Q at approximately x ¼ 4.38mfor selected cases were calculated from the masscollected using the sand traps. Profiles of the sandflux q at each height were obtained for each case asdescribed in Eq. (7). The total sand fluxes calculatedfrom these curves with Eq. (8) are given inTables 12–17. The measurements indicate that allfour soils had substantial amounts of saltatingparticles. Q for both cases of enhanced upwindsaltation decreased slightly in comparison to thisvalue for the saltating sand alone. The ‘‘loamy’’soils had the lowest Q, but only slightly lower thanthe sands. Q are nearly equivalent for all cases, andincrease with u*. The slightly differing sand fluxrates between soils at a given u* can be directlyattributed to the dust content again. A likely

mechanism for this result is that ‘‘sand on sand’’impacts are more elastic and movement is facili-tated, while ‘‘sand on soil’’ collisions are moreplastic and impede the momentum of the particlesthat are moving through the soil. Shao et al. (1993)note that ‘‘the sand particles are coated by dust andthe cohesive forces between grains are greatlyenhanced and the coefficient of restitution (crudely,the elasticity) of the bed is greatly reduced’’.

Finally, a ratio of horizontal PM10 flux FH to thetotal soil flux qtot (sand and dust) was calculated asshown in Fig. 13. Horizontal PM10 flux becomes alarger ratio of the total mass suspended as thefriction velocity increases. High energy impacts ofsaltating particles are likely to play the role ofabrading and breaking agglomerates. There is alsosignificant difference between sand and the emissivesoils as represented by the two trend lines. This ratiois very similar to the vertical flux to horizontal massratio.

3.5. Vertical flux to horizontal mass flux ratio

The vertical dust fluxes Fa estimated from thewind-tunnel concentration profiles along with qtot

ARTICLE IN PRESS

Table 12

Calculated total sand flux Q, horizontal PM10 flux FH, and ratio of FH to the total mass flux qtot for the Pipe Line Soil (North Soil) from

the wind-tunnel measurements

Desig. u* (m s�1) Uref (m s�1) FH (PM10) (gm�1 s�1) Fa (PM10) (gm

�2 s�1) Q (gm�1 s�1) FH/qtot Fa/qtot (m�1)

Soil #1 0.48 9.6 0.00015 0.00002 0.33 0.00044 0.00006

Soil #1 0.49 10.4 0.00045 0.00011 1.23 0.00037 0.00009

Soil #1 0.57 10.3 0.00077 0.00028 1.12 0.00069 0.00025

Soil #1 0.71 11.3 0.02002 0.00900 17.20 0.00120 0.00052

Soil #1 0.76 11.2 0.02210 0.01013 13.17 0.00170 0.00077

Soil #1 1.12 14.2 0.08590 0.04120 51.44 0.00170 0.00080

Table 13

Calculated total sand flux Q, horizontal PM10 flux FH, and ratio of FH to the total mass flux qtot for the North Sand (North Soil) from the

wind-tunnel measurements



Soil #2 0.47 9.1 0.00023 0.00009 1.72 0.00013 0.00005

Soil #2 0.48 8.9 0.00011 0.00002 1.41 0.00008 0.00002

Soil #2 0.53 10.3 0.00060 0.00020 3.93 0.00015 0.00005

Soil #2 0.53 10.1 0.00045 0.00025 2.92 0.00015 0.00008

Soil #2 0.59 10.6 0.00168 0.00049 11.64 0.00014 0.00009

Soil #2 0.59 10.1 0.00238 0.00109 6.73 0.00035 0.00007

Soil #2 0.65 11.3 0.00196 0.00051 5.36 0.00037 0.00010

Soil #2 0.76 11.6 0.00526 0.0033 31.42 0.00017 0.00011

Soil #2 0.85 12.9 0.00517 0.00302 33.52 0.00015 0.00009

Soil #2 0.98 13.0 0.00560 0.00381 31.69 0.00018 0.00012

Soil #2 1.00 13.0 0.01480 0.00702 34.40 0.00043 0.00020

Table 14

Calculated total sand flux Q, horizontal PM10 flux FH, and ratio of FH to the total mass flux qtot for the enhanced saltation of north soils

from the wind-tunnel measurements



Soil #2 over #1 0.44 9.2 0.00067 0.00023 1.76 0.00038 0.00013

Soil #2 over #1 0.53 10.0 0.00438 0.00265 6.03 0.00073 0.00044

Soil #2 over #1 0.69 11.8 0.02270 0.01190 17.78 0.00127 0.00067

Soil #2 over #1 0.90 12.8 0.05110 0.03050 30.45 0.00168 0.00100

Table 15

Calculated total sand flux Q, horizontal PM10 flux FH, and ratio of FH to the total mass flux qtot for the Dirty Socks Sand (South Soils)




Soil #3 0.56 9.9 0.00011 0.00004 0.47 0.00023 0.00007

Soil #3 0.67 11.2 0.00710 0.00200 23.27 0.00031 0.00009

Soil #3 0.68 11.5 0.00840 0.00288 30.94 0.00027 0.00009

Soil #3 0.75 11.4 0.00340 0.00086 16.15 0.00021 0.00005

Soil #3 0.79 12.9 0.01170 0.00319 42.03 0.00028 0.00008

Soil #3 0.84 12.6 0.01130 0.00389 39.76 0.00028 0.00010

Soil #3 1.01 12.5 0.01600 0.00709 41.35 0.00039 0.00017

Soil #3 1.02 13.0 0.01320 0.00529 39.00 0.00034 0.00014

Soil #3 1.09 13.6 0.01630 0.00692 52.38 0.00031 0.00013


ARTICLE IN PRESS

Table 16

Calculated total sand flux Q, horizontal PM10 flux FH, and ratio of FH to the total mass flux qtot for the UCD Fence Soil (South Soil) from

the wind-tunnel measurements


�2 s�1 ) Q (gm�1 s�1) FH/qtot Fa/qtot (m�1)

Soil #4 0.51 8.9 0.00009 0.00002 0.15 0.00057 0.00015

Soil #4 0.67 11.7 0.00220 0.00128 3.30 0.00066 0.00039

Soil #4 0.69 11.6 0.01590 0.00849 7.23 0.00220 0.00117

Soil #4 0.75 12.7 0.04150 0.02150 18.43 0.00225 0.00116

Soil #4 0.86 13.9 0.06020 0.02200 27.33 0.00220 0.00081

Soil #4 0.90 13.8 0.04320 0.02200 19.83 0.00217 0.00111

Table 17

Calculated total sand flux Q, horizontal PM10 flux FH, and ratio of FH to the total mass flux qtot for the enhanced saltation of south soils




Soil #3 over # 4 0.40 9.3 0.00010 0.00013 0.15 0.00065 0.00082

Soil #3 over # 4 0.61 11.1 0.00715 0.00199 8.68 0.00082 0.00023

Soil #3 over # 4 0.71 12.8 0.01970 0.00730 21.32 0.00093 0.00034

Soil #3 over # 4 0.75 12.9 0.01950 0.00831 21.59 0.00090 0.00039

Horizontal Flux Ratios

u∗ (m/s)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

FH /q

tot (

PM

10)

0.00001

0.00010

0.00100

0.01000

0.10000Pipe Line Soil ( Soil #1)North Sand (Soil #2)North Sheet Sim. (Soil #2 over Soil #1)

UCD Fence Soil (Soil #4)Dirty Socks Sand (Soil #3)South Sheet Simulation (Soil #3 over Soil #4)

Fig. 13. PM10 flux to total flux ratios from the wind-tunnel

experiments. Horizontal PM10 flux becomes a larger ratio of the

total mass suspended as the friction velocity increases. High

energy impacts of saltating particles are likely play the role of

abrading and breaking agglomerates. There is also significant

difference between sand and the emissive soils represented by the

two trend lines.


were used to obtain a vertical flux ratio. At thispoint it is important to refer to the results presentedfor Owens (dry) Lake by Gillette et al. (1997a) andNiemeyer et al. (1999). Their plot (presented in bothpapers) specifically shows the vertical flux to total

flux for seven Owens (dry) Lake cases as well as forseveral Western Texas agricultural soils (a compila-tion of many years of research). The wind-tunnelresults compare favorably with the field techniquesof Niemeyer et al. (1999) (sun photometer) andGillette et al. (1997a) (multiple on-lake monitoringsites) as shown in Fig. 14. The ratios are on the sameorder of magnitude for the south lake location. Thewind-tunnel tests designated as ‘‘UCD Fence Soil’’,‘‘South Sheet Simulation’’ and ‘‘Dirty Socks Sand’’correspond to the ‘‘Owens SW’’ field locations.Likewise, the other wind-tunnel cases can becompared to Gillette’s Texas soils; the ‘‘NorthSand’’ corresponding to the ‘‘sand’’ cases and the‘‘North Sheet Simulation’’ corresponding to the‘‘Loamy Sand’’ case. The ratios are similar inmagnitude and behavior for the same soil types.The values of the Total Sand Flux Q, HorizontalPM10 Flux FH, and Vertical PM10 Flux Fa are givenin Tables 12–17.

4. Conclusions

The Saltation Wind Tunnel (SWT) at the Uni-versity of California at Davis was employed toperform a series of experiments aimed at establish-ing a methodology for determining fugitive dustemission rates. As a case study, this methodologywas used for quantifying the conditions for highemissions from Owens (dry) Lake soils. Four soils

ARTICLE IN PRESS

PM10 Vertical Flux to Total Sand Flux

u∗ (m/s)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Fa

/qto

t (P

M10

)

0.00001

0.00010

0.00100

0.01000

0.10000Pipe Line SoilNorth SandNorth Sheet Sim.UCD Fence SoilDirty Socks SandSouth Sheet Sim.

Owens SW (Gillette)Owens SW (Niemeyer)

Texas Sand (Gillette Data)

Emissive Soils

Sand

Fig. 14. Vertical PM10 flux to total flux ratios estimated from the

wind-tunnel experiments are shown. Vertical PM10 flux becomes

a larger ratio of the total mass suspended as the friction velocity

increases. Two field studies at Owens (dry) Lake, Niemeyer et al.,

(1999) and Gillette et al., (1997a), show comparable numbers for

the Owens Lake soils as well. Data for Texas sands also presented

compare well with the sands at Owens Lake.


believed to be causal in the fugitive dust storms weretargeted and emission rates obtained for varyingsurface conditions. Though the variables are nu-merous, the wind tunnel proved to be a significantaid in quantifying conditions for high emissions.First order conditions (wind, surface roughness, andsoil types) similar to those at Owens (dry) Lake werematched and emission rates established for each ofthe four soils.

A ratio of horizontal PM10 flux FH to the totalsoil flux qtot (sand and dust) was calculated andplotted for all the cases. The wind-tunnel results forthis ratio as well as the vertical flux ratio comparefavorably with the field techniques of Niemeyeret al. (1999) (sun photometer) and Gillette et al.(1997a) (multiple on-lake monitoring sites) atOwens (dry) Lake. The wind tunnel, thus, providescomparable data to two separate field techniquesand can be applied systematically without relying onthe variability of the wind, making this techniqueinvaluable in assessing the potential emissions offugitive dusts.

Lastly, fetch effect studies were conducted toobserve the development of emissions along the testbed. In all cases, for the soils without upwindsaltation, the emissions reached an equilibrium

between 2.65 and 4.38m for the higher velocitiestested. A suggested mechanism is that at the nearsurface, the air has become saturated and emissionsare suppressed at the surface even though there isstill upward entrainment of existing particles. Whenupwind ‘‘sand’’ saltation is introduced, the satura-tion condition disappears as the vertical flux isactive due to ballistic sand impacts enhancing thedynamics of the near surface of the soil. Sandsaltation again plays a dynamic role in enhancingemissions. The equilibrium condition is thus pri-marily the result of near surface characteristics ofthe soil in which severe loading near the surfaceprevents the same increases in emissions seen earlierin the fetch. The height limitation of the wind tunnelcan also aid in reaching this saturation. The fetcheffect is critical in estimating emissions at Owens(dry) Lake as well. This study shows that it isunlikely that PM10 emissions rate will continuallyincrease across an erodible surface, but may insteadreach saturation states. This is contrary to the Oweneffect as described in Gillette et al. (1997b), whichstates that the horizontal mass flux or saltationshould increase over the fetch, and as a result thedust emissions should also increase. However, inGillette et al. (1997a), an increasing measuredsaltation rate was measured along a 100m fetch atOwens (dry) Lake, but dust emissions decreased inthe first 50m before recovering. Thus, the wind-tunnel experiments may be exposing a near-surfacesaturation effect that may also occur on the playa aswell.

Acknowledgments

The California Air Resources Board (CARB)funded parts of this research under the InteragencyAgreement 97-718. In addition, the Great BasinUnified Air Pollution Control District (GBUAPCD)was supportive of this research effort and grantedaccess to Owens (dry) Lake.

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